Download Podcast script Chapter 6: Skeleto-muscular

Ireland 3e Chapter Podcast, Chapter 6
The Skeleto-muscular System
Movement through the environment is accomplished by the
coordinated actions of many systems, the main two being the skeletal
system and the muscular system. Studying these systems side by
side provides an easy way to understand not only their individual
actions, but also their integration causing our fluid and often-effortless
motion.
Despite the decreased demands put on our skeleto-muscular system
by the daily needs of our technologically enhanced lifestyle, humans
still need the coordinated action of bone and muscle to move, remain
warm, and stand upright. The underpinnings of this coordinated action
is the skeletal system. Composed of connective tissue, the human
skeleton includes bones, ligaments and cartilage. Functions of the
skeletal system include support, protection, blood cell production, and
mineral storage. The muscles are hung on this connective tissue
framework, and when stimulated, they provide the smooth movement
of that bony framework. Functions of the muscular system include
movement, heat production and protection of underlying structures.
The skeletal system is made of strong, yet light supporting bones.
These were formed through either endochondral or intramembranous
ossification. Endochondral ossification is the process by which the long
bones of the body are formed. Before birth, a cartilage model of the
arm bones, the leg bones, the ribs, and other long bones appears. As
blood vessels enter these models, the cartilage becomes ossified.
Osteoblasts, or immature bone cells, appear and begin to lay down the
calcium matrix of the bone. As these cells continue to form bony
matrix, they become the osteocytes of mature bone. At the ends of
the long bones, a second nutrient artery penetrates the cartilage
model and a second smaller ossification site appears. Between the
main ossification area in the middle of the developing bone and the
smaller ossification sites at the ends, a bit of cartilage is left. This
cartilage forms the growth plate, allowing the bone to grow in length
until adult size is reached. As bone develops, it takes one of two
forms. Compact bone is dense, tough and forms the outer portions of
most bones. The inner structure of this tissue is a series of parallel
osteons running the length of the bone. Blood vessels and nerves are
found within the center of each microscopic osteon. Spongy bone, on
the other hand, is far lighter and less dense. It occupies the areas at
the ends of the long bones, and in the center of many of the flat and
irregular bones. Red bone marrow is found within the spaces of the
spongy bone. Although bone seems hard and unyielding, it is
constantly remodeled. Osteoclasts remove calcium from the matrix of
bone, replenishing blood calcium levels. This continues until blood
calcium levels are high, at which time osteoblasts remove the calcium
from the blood and deposit it in the bones. When new stress is placed
on the skeletal system, by lifting weights for example, osteoblasts lay
down new matrix to support these new demands. If demands on the
bones lessen, osteoclasts remove the now-unnecessary excess matrix
of the bones. Repair of broken bones is carried out in a similar
fashion, with osteoblasts working to restore the missing or damaged
bone.
The human skeletal system is composed of 206 bones, grouped into
the axial and appendicular skeleton. The axial skeleton includes those
bones found in the axis of the body; the skull, vertebrae, ribs and
sternum. The appendages (arms and legs, hands and feet) along with
the girdles that hold them to the axial skeleton comprise the
appendicular skeleton. Each of the 206 bones of the body can be
classified based on its shape as well. Bones are considered long,
short, flat, irregular, sesamoid or wormian, depending on their shape
and location.
Joints are articulations between bones. There are three main types of
joint in the human body. Immovable joints link the bones of the skull,
semimovable joints are found between bones that show limited
motion, as the two halves of the pelvic girdle. The most common joint
in the body is the synovial, or freely movable joint. Synovial joints
have fluid between the two bones, are surrounded by a joint capsule,
and may have bursae and menisci associated. The muscular system
provides movement of the bones at these joints, facilitating our
movement through the environment.
Skeletal muscle is contractile tissue that is under voluntary control.
The muscles of the body are set up so that they pull against one
another in antagonistic pairs, to fluidly move the bones of the
skeleton. The bone that moves when a muscle contracts is referred to
as the insertion of that muscle. The bone that remains stationary
when the muscle contracts is the origin of that muscle. Within the
muscle, there are layers of connective tissue surrounding groups of
muscle cells. The outermost covering of the muscle is the fascia or
epimysium. Within the organ itself, the perimysium surrounds groups
of muscle cells, and the endomysium covers individual cells. These
cells are composed of stacks of contractile units called sarcomeres.
The proteins actin and myosin are found within these sarcomeres, in
strictly organized fashion. Muscle contraction is understood to be an
interaction between the heads of the myosin fibers and the exposed
active site of the actin fibers. The sliding filament theory describes
this motion, demonstrating that the actin filaments are pulled toward
the center of the sarcomere, sliding over their myosin counterparts.
Calcium ions are the trigger to contract, so that when calcium is
released within the muscle cell the sarcomeres shorten. Specifically
calcium binds to the actin filaments, exposing the active site. Myosin
heads then reach up and grab that active site, using ATP and bending
toward the center of the sarcomere as they make contact with the
actin. This infinitesimally small shift toward the center of the
sarcomere is repeated millions of times over in a single muscle
contraction, resulting in the gross movements we associate with
muscular activity. It is incredible really, that such a small change
within a cell causes the strong movements we excitedly cheer on
during athletic competitions.
All of that sliding of filaments requires ATP. Muscles maintain a small
store of ATP directly in their tissues. Creatin phosphate is also used to
provide rapid energy for sustained contractions. Ultimately, oxygen
must be delivered to working muscles in order for contractions to
continue. If oxygen demands are not met, aerobic metabolism cannot
continue. In this case, a less efficient form of energy production can
help generate ATP for the muscles. The anaerobic pathway causes a
build-up of lactic acid though, which hinders muscle contraction.
Oxygen must be “repaid” to remove that lactic acid. Some muscle
cells have a large supply of glycogen within their membrane, and are
able to provide a strong sharp burst of energy. These fast glycolytic
fibers are strong, but tire quickly. Slow oxidative fibers have a good
blood supply and many mitochondria. They are excellent at
aerobically producing energy, but cannot give an initial burst of speed.
These muscle cells are useful as we continue activity for long periods
of time. The relative amount of each fiber type you have in your
muscle determines the overall activity of that muscle – fast and
strong, or efficient in the long haul. An interesting thing about
muscles is that as you use them, they get more efficient. Blood flow
increases, energy production is enhanced, and additional sarcomeres
are added, resulting in muscle tone. In toned muscles, the
sarcomeres remain in a “ready” state, using small but noticeable
amounts of ATP all day long. Toned muscles appear firm, generate
more heat for the body, and use more calories than do untrained
muscles.
The skeleto-muscular system is an excellent example of the
integration of form and function, providing both support, strength and
movement. Although often less obvious than the interaction of these
two systems, every system in the body is part of an amazingly
integrated and efficient whole.